Decision Support Systems (DSS) have been taken as hopeful support tools for decision making for more than 20 years. There are a lot of literatures on DSS, but most of them are not so practical as the designers expected. This paper points out the crux of this situation and argues that the research on DSS should pay some more attention to the decision making activities before the model using stage. A method named "Problem Situation Decomposing Graph (PSDG)" is presented in this paper for helping the decision maker(DM) elicit the decision making problems. A PSDG is an acycle AND/OR logical directed graph, and which includes all the factors affecting the problem situation based on the DM's knowledge. The logical nodes and parameter determining methods in PSDG can reflect the DM's decision making style. This paper introduces some basic concepts of PSDG, discusses some of its characteristics, and proposes a logical adjacency matrix for PSDG representation and analysis.